Method and apparatus for the formation of particles

ABSTRACT

The invention provides a method for forming particles of a substance, by co-introducing into a particle formation vessel, in which the temperature and pressure are controlled, of a supercritical fluid; a solution or suspension of the substance in a first vehicle; and a second vehicle which is both substantially miscible with the first vehicle and substantially soluble in the supercritical fluid, in such a way that dispersion of the solution or suspension and the second vehicle, and extraction of the vehicles, occur substantially simultaneously and substantially immediately on introduction of the fluids into the vessel, by the action of the supercritical fluid. Preferably the solution/suspension of the substance is introduced separately from the second vehicle, in such a way that contact between the solution/suspension and the second vehicle occurs either substantially simultaneously with, or immediately before, their dispersion by the supercritical fluid and extraction of the vehicles by the supercritical fluid. The method allows a high degree of control over the size, shape, crystalline form and other physico-chemical properties of the particulate product. The invention also provides apparatus for carrying out such a method, using a coaxial nozzle to introduce the fluids into the particle formation vessel, and a particulate product made using the method or the apparatus.

FIELD OF THE INVENTION

This invention relates to the controlled formation of particulateproducts using supercritical fluids. It provides a method and apparatusfor the formation of a substance in particulate form, and theparticulate product of such a method.

BACKGROUND TO THE INVENTION

The use of supercritical fluids (SCFs) and the properties thereof havebeen extensively documented; see for instance J. W. Tom and P. G.Debenedetti. “Particle Formation with Supercritical Fluids—A Review”, J.Aerosol. Sci. 22 (5), pp 555-584 (1991). Briefly, a supercritical fluidcan be defined as a fluid at or above its critical pressure (Pc) andcritical temperature (TC) simultaneously. Such fluids have been ofconsiderable interest, not least because of their unique properties.These characteristics include:

-   High diffusivity, low viscosity and low surface tension compared    with liquids.-   High compressibility compared with the ideal gas implies large    changes in fluid density with only slight changes in pressure, which    in turn results in highly controllable salvation power.    Supercritical fluid densities typically range from 0.1-0.9 g/ml    under normal working conditions. Thus, selective extraction with one    supercritical fluid is possible.-   Many supercritical fluids are normally gases under ambient    conditions, which eliminates the evaporation/concentration step    needed in conventional liquid extractions.-   Most of the commonly used supercritical fluids create non-oxidising    or non-degrading atmospheres for sensitive and thermolabile    compounds, due to their inertness and the moderate temperatures used    in-   routine working conditions. Carbon dioxide is the most extensively    used SCF due to its cheapness, non-toxicity, non-flammability and    low critical temperature.

These characteristics have led to the development of several techniquesof extraction and particle formation utilising supercritical fluids. Inparticular, two particle formation methods have been identified.

“Rapid expansion of supercritical solution” (RESS) (see. for instance.J. W. Tom and P. G. Debenedetti, supra) involves the dissolution of thesolute of interest in a supercritical fluid, followed by rapid expansionof the resulting supercritical solution to atmospheric pressure,resulting in the precipitation of solute particles.

“Gas anti solvent” (GAS) recrystallisation (P. M. Gallagher et al.“Supercritical Fluid Science and Technology”, ACS Symp. Ser., 406, p 334(1989)) is particularly useful in situations when the solute of interestdoes not dissolve in, or has a very low solubility in, a supercriticalfluid or a modified supercritical fluid. In this technique, the soluteis dissolved in a conventional solvent. A supercritical fluid such ascarbon dioxide is introduced into the solution, leading to a rapidexpansion of its volume. As a result, the solvent power decreasesdramatically over a short period of time, triggering the precipitationof particles.

The concept of spraying liquid mixtures into supercritical fluids suchas carbon dioxide, or vice versa, has also been employed in solventextraction procedures for a decade (see for instance R. J. Lahiere & J.R. Fair in Ind. Eng. Chem. Res., 26, pp 2086-2092 (1987)).

More recently, U.S. Pat. No. 5.043,280 describes a method formanufacturing a preparation comprising a substance, such as a medicallyuseful substance, and a carrier, such as a pharmaceutically acceptablecarrier, which avoids of lacks a solvent residue, or at least reducesthe solvent residue to a toxicologically harmless amount. The methodessentially involves the use of a fluid, at a supercritical state whenintroduced into a spray tower, to extract a solvent from sprayedsolution(s) of a substance and a carrier, to form a sterile productcontaining the substance embedded in the carrier. It should be noted,however, that the method has no means for controlling the physicalproperties of the particulate product formed.

In many fields, and especially in the fields of pharmaceuticals,photographic materials, ceramics, explosives and dyes, there is a needfor techniques whereby a particulate product may be obtained withconsistent and controlled physical criteria, including particle size andshape, quality of the crystalline phase, chemical purity and enhancedhandling and fluidising properties.

In addition, it would be advantageous to be able to prepare micron-sizedparticles directly, without the need to mill products to that sizerange. Such milling can lead to associated problems such as increasedstatic charge and enhanced particle cohesiveness, as well as reducedproduct yield.

A further method for forming particulate products using supercriticalfluids has been described more recently in our co-pending PCT patentapplication. no. PCT/GB94/01426 of 30 Jun. 1994. which claims priority fox UK patent application no. 9313650.5 of 1 Jul. 1993 and was publishedas WO-95/01221. In the method described in that, patent application, asubstance to be produced in particulate form is dissolved or suspendedin an appropriate vehicle. The resulting solution or suspension is thenco-introduced into a particle formation vessel with a supercriticalfluid (preferably through a co-axial nozzle), in such a way thatdispersion and extraction of the vehicle occur substantiallysimultaneously by the action of the supercritical fluid, andsubstantially immediately on introduction of the fluids into the vessel.The pressure and temperature inside the particle formation vessel arecarefully controlled during this process

This method allows a high degree of control over conditions such aspressure and temperature and fluid flow rates, at the exact point whereparticle formation occurs (i.e. at the point where the vehicle isextracted into the supercritical fluid). It therefore allows greatcontrol over the size and shape of the particles formed, and over otherphysical and/or chemical properties of the particles, including thepolymorphic form where several are possible. The method is thus idealfor producing particles for use in fields where such high levels ofcontrol are necessary, for instance in the manufacture ofpharmaceuticals, photographic materials, ceramics, etc . . . The methodobviates the need for milling particulate products to a desired sizerange, thus eliminating the disadvantages of increased static charge,enhanced particle cohesiveness and reduced product yield, describedabove.

The applications of this and other particle formation techniques usingsupercritical fluids are, however, limited. The vehicle chosen must besoluble in the chosen supercritical fluid. Also, the substance itself,from which particles are to be formed, must be capable of dissolution,or at least suspension, in the chosen vehicle. It is not always easy toselect a vehicle that can both dissolve the substance and also itselfdissolve in the supercritical fluid being used (in practice, usuallycarbon dioxide).

An example of a situation in which such problems arise is thepreparation of lactose. Lactose is commonly used as a carrier forpharmaceuticals, in tablets and capsule formulations and in particularfor drugs to be delivered by inhalation methods. It thus needs to beprepared in the form of particles which have, amongst othercharacteristics, a narrow size distribution, a high purity and anappropriate particle shape.

However, lactose has very low solubility in conventional organicsolvents which might be used with supercritical carbon dioxide in knownparticle formation techniques. Lactose dissolves readily in water, butwater will not dissolve in supercritical carbon dioxide. It has thus,previously, been very difficult to form lactose particles directly fromaqueous solution using known supercritical fluid techniques (includingthat described in WO-95/01221), since the supercritical fluid (typicallycarbon dioxide) would not extract water from the aqueous solution, orwould do so so slowly as to be impractical. Nevertheless, it would begenerally desirable to be able to form lactose particles in thecontrolled manner that supercritical fluid techniques (in particularthat described in WO-95/01221) would allow.

It is generally known that other sugars and many amino acids andproteins suffer from similar disadvantages to that of lactose, ie. theyhave very low solubility in organic solvents and supercriticalfluids/modified supercritical fluids (see Stahl et al. “Dense GasExtraction on a Laboratory Scale: A Survey of some Recent Results”,Fluid Phase Equilibria. 10, p 269, 1983) and cannot therefore be formedinto particles using former supercritical fluid particle formationtechniques (RESS in particular). Again, as with lactose, it would bedesirable to be able to produce particulate forms of such compounds in acontrolled manner, for instance for use in pharmaceuticals andfoodstuffs.

A related problem arises with many proteins. Although solutions of suchproteins in organic solvents can be prepared, it is generallyundesirable co do so because of the risk of the protein unfolding anddenaturing (see, for instance, K. A. Dill & D. Shortle. Ann. Rev.Biochem., 1991, 60. pp 795-825. especially p 813) Thus, it is difficultif not impossible to prepare particulate products of such proteins, withacceptable biological activity. Using known supercritical fluid particleformation techniques.

There are many other examples of substances which might otherwise beformed into particles using supercritical fluids, but which cannot besufficiently well dissolved or suspended in an appropriate solvent whichwill itself dissolve in a useful supercritical fluid.

There is therefore a need to solve this problem, to allow the use ofsupercritical fluid particle formation techniques (including theextremely effective technique described in WO-95/01221) for substancessuch as lactose and proteins. The present invention sets out toovercome, or at least mitigate, the problem.

STATEMENTS OF THE INVENTION

According to a first aspect of the present invention, there is provideda method for forming particles of a substance, the method comprising theco-introduction into a particle formation vessel, the temperature andpressure in which are controlled, of a supercritical fluid; a solutionor suspension of the substance in a first vehicle; and a second vehiclewhich is both substantially miscible with the first vehicle andsubstantially soluble in the supercritical fluid, in such a way thatdispersion of the solution or suspension and the second vehicle andextraction of the vehicles by the supercritical fluid occursubstantially simultaneously and substantially immediately onintroduction of the fluids into the particle formation vessel.

As will be explained below, contact between the solution or suspensionand the second vehicle may occur either at much the same time as, orslightly before, dispersion and extraction by the supercriticalfluid—the timing will depend on the nature of the substance from whichparticles are to be formed, and the nature of the desired end product.

In other versions of the process, useful or advantageous results maystill be achieved even if the first and second vehicles meetsubstantially before introduction to the vessel.

The substance will typically (although not always) be one which, asdescribed above, is soluble or substantially soluble only in solventswhich are themselves substantially insoluble in the supercritical fluid.It may be a substance which, though soluble in an appropriatesupercritical fluid-soluble solvent, would suffer detrimental effects ifdissolved in that solvent prior to particle formation (for instance, ahydrophilic protein), or be otherwise incompatible with such a solvent.It is preferably substantially soluble, however, in the first vehicle.

As used herein, the term “supercritical fluid” means a fluidsubstantially at or above its critical pressure (Pc) and criticaltemperature (Tc) simultaneously. In practice, the pressure of the fluidis likely to be in the range (1.01-7.0)Pc, and its temperature in therange (1.01-4.0)Tc.

The term “vehicle” means a fluid which is able to carry a solid orsolids in solution or suspension. Each vehicle may be composed of one ormore component fluids. Both vehicles may be substantially soluble in thechosen supercritical fluid, although it is only essential that thesecond vehicle has this characteristic.

As used herein, the term “supercritical solution” means a supercriticalfluid together with a vehicle or vehicles, as defined above, which ithas extracted and dissolved. The solution should itself still be in thesupercritical state.

The term “dispersion” means the formation of droplets, or otheranalogous fluid elements, of the solution or suspension and/or thesecond vehicle.

The term “substance” includes substances in a single-component ormulti-component (e.g. an intimate mixture, or one component in a matrixof another) form.

The present invention relies on the dissolution or suspension of thesubstance of interest in a first vehicle, in which it is preferablysubstantially soluble, with “dilution” of the resultant solution orsuspension, either beforehand or substantially simultaneously with itsdispersion by the supercritical fluid, in a (conveniently) relativelylarge amount of a second vehicle specially chosen to be soluble in thesupercritical fluid used for particle formation. On “mixing” of thesubstance and first vehicle, with the second vehicle, at their point ofcontact, hydrogen-bonding and/or similar interactions (eq. dipole-dipoleinteractions) are thought to form between molecules/ions of the firstand second vehicles, such that on contact with the supercritical fluid,the relatively small amount of the first vehicle is able to dissolveinto the supercritical fluid with the second vehicle, ie. the twovehicles may effectively be extracted together by the supercriticalfluid to form a supercritical solution.

It is thus preferred that the flow rates of the two vehicles, onintroduction into the particle formation vessel, be such as to create anexcess of the second vehicle over the first at their moment of contact.

The advantage of the method of the invention is that it allows thepreparation of particles, using a supercritical fluid technique, ofsubstances which could not otherwise be used in such a technique becauseof their very low solubility in, or incompatibility with, the necessarysolvents. It therefore considerably broadens the applications availablefor supercritical fluid particle formation techniques.

Moreover, because “mixing” of the two vehicles may occur substantiallysimultaneously with, or immediately prior to, dispersion and extractionby the supercritical fluid, a simple solution or suspension of thesubstance of interest in the first vehicle for instance, an aqueoussugar or protein solution) may be introduced into the particle formationvessel, without the need to prepare complex two-vehicle systemsbeforehand.

This also means that the substance of interest need not be mixed withany incompatible vehicle(s) until immediately prior to, or at, particleformation. Thus, for instance, a hydrophilic protein may be introducedinto the particle formation vessel in aqueous solution, and only comesinto contact with the second (usually organic) vehicle at or just beforethe point of dispersion by the supercritical fluid, thus minimising therisk of protein denaturing prior to particle formation.

The method of the invention also retains all the advantages of themethod described in WO-95/01221. It provides the opportunity formanufacturing dry particulate products with highly controlled particlesize and shape, by offering control over the working conditions,especially the pressure and temperature, at the exact point of particleformation. Such an improved control eliminates significant pressurefluctuation across the particle formation vessel and ensures a moreuniform dispersion of the solution or suspension and the second vehicleby the supercritical fluid, resulting in a narrow size distribution forthe fluid elements (eg. droplets) formed during the particle formationprocess. There is little or no chance chat the dispersed fluid elementswill reunite to form larger elements since the dispersion occurs by theaction of the supercritical fluid itself, which also ensures thoroughmixing of the solution or suspension and the second vehicle whilstrapidly removing the vehicles from the substance of interest to causeparticle formation.

The simultaneous co-introduction of the solution or suspension, thesecond vehicle and the supercritical fluid, into a vessel inside whichpressure and temperature are controlled, allows a high degree of controlof operating parameters such as temperature, pressure and fluid flowrates, at the exact point when the fluids come into contact with oneanother and at the point of actual particle formation. Importantly, thesupercritical fluid then acts both to disperse and to extract the twovehicles, as well as to ensure their thorough mixing—because of this,controlling the relative flow rates of the fluids, into the particleformation vessel, allows accurate control over the size of the droplets(or other fluid elements) formed on dispersion of the vehicles, andhence of the particles formed substantially simultaneously by extractionof the vehicles into the supercritical fluid.

Through the pressure and temperature control, supercritical conditionsare ideally maintained in the particle formation vessel at all times.The flow rate of the supercritical fluid relative to those of the otherfluids, and its pressure and temperature, should be sufficient to allowthe supercritical fluid to accommodate the vehicles (generally, thevehicles will represent no more than around 5

v/v of the supercritical fluid), so that it can extract them from themixture and hence cause particle formation. Careful selection of suchoperating conditions ensures the existence of only a singlesupercritical phase during the entire particle Formation process,allowing improved control over particle characteristics andsubstantially eliminating the risk of residual solvent in theparticulate product.

Further advantages, for particles formed using a method according to thepresent invention, include good control over the quality of thecrystalline and polymorphic phases, since the particles will allexperience the same stable conditions of temperature and pressure whenformed, as well as the potential for enhanced purity. This latterfeature can be attributed to the high selectivity of supercriticalfluids under different working conditions, enabling the extraction ofone or more impurities from the vehicle containing the substance ofinterest.

Moreover, the co-introduction of the solution or suspension, the secondvehicle and the supercritical fluid, leading to simultaneous dispersionand particle formation, allows particle formation to be carried out, ifdesired, at temperatures at or above the boiling point of either or bothof the vehicles, something not possible using conventional particleformation techniques. This enables operation in temperature and pressuredomains which were previously inaccessible, which in turn can allow theformation of products, or particular forms of products, that previouslycould not have been achieved. This, together with the high degree ofcontrol of the operating conditions made possible by the presentinvention, means that its uses could be extremely wide-ranging and itsversatility of value in many fields.

The substance used in the method of the invention may be any substancewhich needs to be produced in particulate form. It typically (althoughnot always) either is or comprises a substance which is soluble, orsubstantially soluble, only in solvents which are themselvessubstantially insoluble in the supercritical fluid chosen for use in themethod, or which is incompatible with supercritical fluid-solublesolvents. Examples of such substances, where the supercritical fluid iscarbon dioxide, may include lactose and other sugars, proteins,hydrophilic enzymes, inorganic materials such as for use as dyestuffs).etc . . . However, it is to be understood that any substance from whichparticles are to be formed may be used in the method of the presentinvention.

In one particular embodiment of the invention, the substance from whichparticles are to be formed is for use in or as a pharmaceutical.However, the end product may in fact be any desired particulate product,for instance a product of use in the ceramics, explosives orphotographic industries; a foodstuff; a dye; etc . . . In each case, theprinciple behind the method of the invention remains the same; thetechnician need only adjust operating conditions in order co effectproper control over the substance being used.

The substance may be in a single or multi-component form (eg. it couldcomprise an intimate mixture of two materials, or one material in amatrix of another, or one material coated onto a substrate of another,or other similar mixtures). The particulate product, formed from thesubstance using the method of the invention, may also be in such amulti-component form—as described below, such products may be made fromsolutions or suspensions containing only single component startingmaterials, provided the solutions/suspensions are co-introduced with thesupercritical fluid in the correct manner. The particulate product mayalso be a substance formed from an in situ reaction (ie. immediatelyprior to, or on, dispersion by the supercritical fluid) between two ormore reactant substances carried by the first and second vehicles.

The supercritical fluid may be any suitable supercritical fluid, orinstance supercritical carbon dioxide, nitrous oxide, sulphurhexafluoride, xenon, ethylene, chlorotrifluoromethane, ethane ortrifluoromethane (again, this list is not exhaustive). A particularlypreferred supercritical fluid is carbon dioxide, due to its relativecheapness, non-toxicity, non-flammability and relatively low criticaltemperature.

The supercritical fluid may optionally contain one or more modifiers,for example, but not limited to, methanol, ethanol, isopropanol oractone. When used, the modifier preferably constitutes not more than20%, and more preferably constitutes between 1 and 10%, of the volume ofthe supercritical fluid.

The term “modifier” is well known to those persons skilled in the art. Amodifier (or co-solvent) may be described as a chemical which, whenadded to a supercritical fluid, changes the intrinsic properties of thesupercritical fluid in or around its critical point.

The first and second vehicles may be any appropriate vehicles, and maybe chosen by the skilled man from within his general knowledge. Thechoice of vehicles in any particular case will depend on the nature ofthe substance from which particles are to be formed, on thesupercritical fluid to be used in forming them, and on other practicalcriteria including those governing the desired end product. The choiceof a suitable combination of supercritical fluid, modifier (wheredesired) and vehicles for any desired product will be well within thecapabilities of a person of ordinary skill in the art.

The first vehicle is preferably one in which the substance issubstantially soluble, but may itself be substantially insoluble in thechosen supercritical fluid. The second vehicle must be substantiallymiscible with the first, and substantially soluble in the chosensupercritical fluid. For example, where the substance is lactose and thesupercritical fluid carbon dioxide, the first vehicle might be water andthe second ethanol.

The tow vehicles should be chosen on the basis of their polarities,functionalities and other considerations, so that for instance onevehicle contains functional groups that can hydrogen-bond with an acidicproton of the other vehicle, or can otherwise interact with functionalgroups contained in the other vehicle. Such interactions help to promotethe extraction of the first vehicle into the supercritical fluidtogether with the second.

In the method of the invention, the substance of interest and the firstvehicle may be substantially polar, the second vehicle then beingsubstantially non-polar and both vehicles being substantially misciblein all proportions and preferably substantially soluble in thesupercritical fluid.

In contrast, a substantially non-polar substance may be dissolved in asubstantially non-polar first vehicle, the second vehicle then beingsubstantially polar and both vehicles being substantially miscible inall proportions and preferably substantially soluble in thesupercritical fluid.

These two sets of conditions are of particular use in a version of theinvention in which the second vehicle acts as an anti-solvent for thesubstance of interest, ie. the substance is substantially insoluble inthe second vehicle. This version of the invention will be described inmore detail below.

It is to be understood that throughout this specification, the terms“first vehicle” and “second vehicle” each encompass a mixture of two ormore fluids which together have the necessary solubilising, miscibilityand polarity characteristics.

As mentioned previously, where is preferably an excess of the secondvehicle at the point of its contact with the first. Typically, theamount of the first vehicle used will be the minimum possible to solvatethe substance so as to create a single phase solution. This ispreferably less than or equal to about 30%, more preferably less than orequal to about 10%, of the total amount of the first and secondvehicles.

The amounts or the vehicles used, and their relative flow rates, mayalso depend on whether it is intended that some of the vehicle(s) remainin the final particulate product. For instance, if the first vehiclewere water, then the amount used, could affect whether the substancewere precipitated in an anhydrous form, or in the form of itsmonohydrate, dihydrate, or whatever. Thus, the concentration of thefirst, or indeed the second, vehicle in the eventual mixture of vehiclesmay be used to control “doping” of the final particulate product withvehicle “impurities”. The invention allows a high degree of control overthe residual vehicle content of the final particulate product.

In certain cases, the amount of the first vehicle (for instance, water)used may also determine which crystalline form of the substance isformed on treatment with the supercritical fluid.

In a preferred embodiment of the invention, the supercritical fluid, thesolution or suspension and the second vehicle are co-introduced into theparticle formation vessel with concurrent directions of flow, morepreferably with substantially coaxial directions of flow, such as byusing a nozzle of coaxial design. Such a nozzle has an outlet endcommunicating with the interior of the particle formation vessel, andtwo or more coaxial passages which terminate adjacent or substantiallyadjacent to one another a the outlet end, at least one of the passagesserving to introduce a flow of the supercritical fluid into the particleformation vessel, at least one of the passages serving to introduce aflow of the solution or suspension of the substance in the first vehicleand at least one of the passages serving to introduce a flow of thesecond vehicle.

The nozzle is preferably of the type which allows “pre-filming” or“sheathing” of at least one of the fluids to occur, immediately prior toits contact with the other fluid(s). Note that this is not the same ascreating a “jet” or “string” of one fluid to be broken up by anotherfluid. Ideally, the nozzle can be used to cause pre-filming of thesolution or suspension and/or of the second vehicle, immediately priorto their dispersion by the supercritical fluid. This means that thedimensions of the nozzle passages, and the relative positions of theiroutlets, must be such that a fluid entering through one passage isformed, as it reaches the outlet of that passage, into a thin film orsheath of fluid, by its contact with, say, the lip of an adjacentpassage outlet. This film or sheath can then be stretched, andultimately dispersed into separate fluid elements, when it comes intocontact with an oncoming stream of the supercritical fluid in anothernozzle passage. Clearly, the thickness of the film or sheath, and hencethe sizes of the fluid elements formed on dispersion, will depend to alarge extent on the relative flow rates of the fluids, and also on thenozzle passage dimensions.

The use of such an inlet device ensures no contact between the formedparticles and the vehicles around the nozzle tip area, which contactwould reduce control of the final product size and shape. Extra controlover the size of the dispersed vehicle fluid elements, in addition tothat provided by the nozzle design, may be achieved according to thefirst aspect of the invention by controlling the flow rates of thesupercritical fluid, the solution/suspension and the second vehicle intothe particle formation vessel. At the same time, the ability to retainthe formed particles in the vessel eliminates the potential for contactwith the vehicles that might otherwise take place on eventuallydepressurising the supercritical solution. Such contact would affect theshape and size, and potentially the yield, of the product.

Preferably, the opening at the outlet end (tip) of the nozzle will havea diameter in the range of 0.05 to 2 mm, more preferably between 0.1 and0.3 mm, typically about 0.2 mm. The angle of taper of the outlet end(with respect to the main axis of the nozzle) will depend on the desiredvelocity of the fluids introduced through the nozzle; a change in theangle may be used, for instance, to increase the velocity of thesupercritical fluid and hence to increase the amount of physical contactbetween the supercritical fluid and the vehicles. Typically (althoughnot necessarily), the angle of taper will be in the range of about 10°to about 60°, preferably between about 10° and 50°, more preferablybetween about 20° and about 40°, and most preferably about 30°. Thenozzle may be made of any appropriate material, for example stainlesssteel.

In one embodiment of the invention, the nozzle has three coaxialpassages, an inner, an intermediate and an outer. This design allowsboth vehicles, and the supercritical fluid, to be introduced separatelyinto the particle formation vessel. However, the nozzle may have anyappropriate number of coaxial passages, some of which may be used tointroduce additional reagents into the particle formation vessel. One ormore of the passages may be used to introduce two or more fluids at thesame time, and the inlets to such passages may be modified accordingly.

The solution or suspension of the substance in the first vehicle may beintroduced through one nozzle passage, and the supercritical fluid andthe second vehicle may be introduced together through another passage.Mixing of the two vehicles then occurs simultaneously with theirdispersion and extraction by the supercritical fluid. This may beeffected using a two-passage nozzle or, using a nozzle having three ormore passages, the solution or suspension may be introduced between aninner and an outer flow of the supercritical fluid/second vehiclemixture, which improves dispersion and mixing by exposing both sides ofthe solution/suspension to the supercritical fluid and second vehicle.

The internal diameters of the coaxial passages may be chosen asappropriate for any particular case. Typically, for a three-passagenozzle, the ratio of the internal diameters of the outer and the innerpassages may be in the range of from 2 to 5, preferably between about 3and 4. The ratio of the internal diameters of the outer and intermediatepassages may be in the range of from 1.2 to 3, preferably between about1.4 and 1.8.

Examples of such coaxial nozzles, and their typical dimensions, areillustrated in FIGS. 3 and 4.

The temperature in the particle formation vessel may be maintained at adesired level (preferably ±0.1° C.) by means of a heating jacket or,more preferably, an oven. The pressure in the particle formation vesselis conveniently maintained at a desired level (preferably ±2 bar at 350bar) by means of an automated back-pressure regulator. It will beappreciated that such apparatus will be readily available from, forexample, manufacturers of supercritical fluid extraction equipment, forinstance, from Jasco Inc., Japan.

Control of parameters such as size, size distribution, shape andcrystalline form in the particulate product will be dependent upon theoperating conditions used when carrying out the method of the invention.Variables include the flow rates of the supercritical fluid and/or thesolution or suspension and/or the second vehicle, the relative amountsof the two vehicles, the concentration of the substance in the firstvehicle, and the temperature and pressure inside the particle formationvessel.

It will also be appreciated that the precise conditions of operationwill be dependent upon the choice of supercritical fluid and whether ornot modifiers are present. Table 1, for instance, lists the criticalpressures and temperatures for some selected fluids: TABLE 1 Fluid Pc(bar) Tc(° C.) carbon dioxide 74 31 nitrous oxide 72 36 sulphurhexafluoride 37 45 xenon 58 16 ethylene 51 10 chlorotrifluoromethane 3929 ethane 48 32 trifluoromethane 47 26

In practice, it may be preferable to maintain the pressure inside theparticle formation vessel substantially in excess of the Pc (forinstance, 100-300 bar for carbon dioxide) whilst the temperature is onlyslightly above the Tc (e.g. 40-60° C. for carbon dioxide).

The flow rates of the supercritical fluid and/or the solution orsuspension and/or the second vehicle, into the particle formationvessel, may be controlled so as to achieve a desired particle size, sizedistribution, shape and/or form. Typically, the ratio of thesolution/suspension flow rate to the supercritical fluid flow rate willbe between 0.001 and 0.2. preferably between 0.001 and 0.1. morepreferably between 0.01 and 0.07, and most preferably around 0.03. Theflow rate of the supercritical fluid, relative to those of the otherfluids, is particularly important because the supercritical fluid actsto disperse the two vehicles. Its flow rate therefore affects the sizeof the droplets or other fluid elements caused by the dispersion, andhence of the particles formed by extracting the vehicles from thosefluid elements.

The method of the invention preferably additionally involves collectingthe particles following their formation, more preferably in the particleformation vessel itself. The method may also involve recovering thesupercritical solution formed on extraction of the vehicles into thesupercritical fluid, separating the components of the solution andre-cycling one or more of those components for future use.

The method is preferably carried out in a substantially continuous, asopposed to batch-wise, manner. Apparatus which can be used to carry outthe process continuously is described below.

According to one particular version of the method of the invention, thesubstance of interest is only sparingly soluble, if at all, in thesecond vehicle. The second vehicle thus acts as an antisolvent for thesubstance, and on contact of the second vehicle with a solution of thesubstance in the first vehicle, the second vehicle causes precipitationof the substance from its solution.

In this case, the second vehicle may contain a “seed” of the substanceof interest, or indeed of any other suitable material (insoluble in thesecond vehicle), to induce nucleation of the substance of interest whenthe second vehicle comes into contact with the solution or suspension ofthe substance in the first vehicle. The seed may be, for example, apharmaceutically acceptable carrier where the substance of interest isfor use in or as a pharmaceutical, or it may itself be apharmaceutically active material, to be coated with a substance, such asa taste-masking agent, which is precipitated out of the first vehicleonto the seed.

When carrying out this version of the invention, the various fluids mustbe introduced into the particle formation vessel in such a way that thesecond vehicle, and the solution or suspension of the substance ofinterest in the first vehicle, contact one another before, andpreferably shortly or immediately before, their contact with and hencedispersion and extraction by the supercritical fluid. The second vehiclecomes into contact with the solution or suspension, and dramaticallyincreases the supersaturation ratio of the resultant mixture, causingnucleation and the formation of embryos or nucleation sites, which canact as centres of crystallisation for the substance of interest. Afterand preferably immediately after this, the mixture is dispersed by thesupercritical fluid, and simultaneously the two vehicles are rapidlyextracted into the supercritical fluid, leading to the formation of adry particulate product.

Dispersion of the mixture of vehicles, which already contains growingparticle embryos, into fluid elements by the supercritical fluid allowsa high level of control over the growth of the particles and hence overtheir ultimate size. By controlling parameters such as the flow rates ofthe solution or suspension, the second vehicle and the supercriticalfluid, and the temperature and pressure inside the particle formationvessel, it is possible to control the size and size distribution of theparticles formed, as well as their shape, morphology and othercharacteristics, to a great degree of accuracy.

When the solution or suspension of the substance in the first vehicle iscontacted with the second vehicle, and the mixture is dispersed by thesupercritical fluid, the second vehicle “diluent” alters the polarity ofthe resultant supercritical solution. This can minimise extraction ofthe substance by the supercritical solution and hence enhance the yieldof the particulate product. The mixing ratio, of the second vehicle withthe solution or suspension of the substance of interest, should ideallybe kept slightly below the supersaturation ratio of thesolution/suspension. In particular when the two are mixed just beforedispersion, because the second vehicle, acting as an anti-solvent forthe substance of interest, can cause precipitation of the substance andeventual blocking of the nozzle or other inlet to the particle formationvessel.

Because of the need for the second vehicle to act as an anti-solvent forthe substance, it is virtually essential in this version of theinvention that the two vehicles have very different polarities. Thus,for instance, the substance and the first vehicle may be substantiallypolar, whilst the second vehicle is substantially non-polar, or viceversa.

Again in this version, the two vehicles are preferably bothsubstantially soluble in the chosen supercritical fluid, and the secondvehicle must be substantially miscible with the first in allproportions.

Such a version of the method is preferably carried out using a coaxialnozzle, as described above, as the means for co-introducing the variousfluids into the particle formation vessel. The nozzle can have at leastthree coaxial passages, to allow the separate introduction of thesolution or suspension, the second vehicle and the supercritical fluidand to allow their contact with one another at the appropriate times.The outlet of at least one of the inner nozzle passages should belocated a small distance upstream (in use) of the outlet of at least oneof its surrounding passages. This allows a degree of mixing to occur,between the solution or suspension and the second vehicle. (and hencealso a degree of particle precipitation) within the nozzle—the solutionor suspension and the second vehicle are introduced through the innerpassage and surrounding passage in question. The supercritical fluid maythen be introduced through an outer passage (ie. one surrounding the twopassages already mentioned), and will contact the mixture, causingdispersion and extraction to occur, downstream of the initial point ofmixing. The relative flow rates of the fluids will determine how soonafter mixing the two vehicles will be dispersed by the supercriticalfluid: typically, very short time intervals will be desired.

A nozzle having more than three coaxial passages may of course be usedin this version of the invention. For instance, a nozzle having four ormore passages may be used to introduce the solution or suspension andthe second vehicle (and preferably to cause their pre-filming), betweenan inner and an outer flow of the supercritical fluid, for instance onethrough the innermost and one through the outermost passage of afour-passage nozzle. Again, the outlet of the inner of the two passagescarrying the solution/suspension and the second vehicle must terminateslightly upstream of the outlet of the outer of these two passages, toallow pre-mixing of the solution/suspension and the second vehicle totake place.

In an alternative version of the method of the invention, the solutionor suspension of the substance in a relatively small amount of the firstvehicle may be added to a relatively large amount of (ie. “diluted in”)the second vehicle, prior to the co-introduction of the mixture into aparticle formation vessel with a supercritical fluid. This may be donewhere there is no incompatibility between the substance of interest andthe two vehicles. Thus, in its broadest aspect, the invention simplyinvolves the use of the first and second vehicles to carry the substanceof interest, and the co-introduction of the vehicles and substance intoa particle formation vessel (the temperature and pressure in which arecontrolled) with a supercritical fluid, in such a way that dispersion ofthe solution or suspension and the second vehicle, and extraction of thevehicles, occur substantially simultaneously and substantiallyimmediately on introduction of the fluids into the particle formationvessel, by the action of the supercritical fluid. Again, the twovehicles “mix” and are extracted together into the supercritical fluid.

Clearly, such a method can be carried out in the same manner as isdescribed above. In particular, an excess of the second vehicle ispreferably used, relative to the amount of the first vehicle used.

According to a second aspect of the present invention, there is providedapparatus for use in carrying out a method according to the firstaspect, the apparatus comprising a particle formation vessel; means forcontrolling the temperature in the vessel at a desired level; means forcontrolling the pressure in the vessel at a desired level; and means forthe co-introduction, into the vessel, of the supercritical fluid, thesolution or suspension of the substance in the first vehicle, and thesecond vehicle, in such a way that contact between the solution orsuspension and the second vehicle occurs either substantiallysimultaneously with, or immediately before, dispersion of the solutionor suspension and the second vehicle by the action of the supercriticalfluid and extraction of the vehicles by the supercritical fluid, andsuch that the dispersion and extraction occur substantiallysimultaneously and substantially immediately on introduction of thefluids into the particle formation vessel.

In general, apparatus for use in carrying out the method of theinvention may comprise any suitable means for co-introducing the fluidsinto the particle formation vessel. However, according to the secondaspect of the invention, the means for the co-introduction of the fluidsinto the vessel comprises a nozzle having an outlet end communicatingwith the interior of the vessel, and at least three (preferably three orfour) coaxial passages which terminates adjacent or substantiallyadjacent to one another at the outlet end, at least one of the passagesserving to introduce a flow of the supercritical fluid into the vessel,at least one of the passages serving to introduce a flow of the solutionor suspension and at least one of the passages serving to introduce alow of the second vehicle, all fluid flows being in substantiallycoaxial directions, and wherein the outlet of at least one of the innernozzle passages is located a small distance upstream (in use) of theoutlet of one of its surrounding passages so as to allow, in use, adegree of mixing to occur within the nozzle, between the solution orsuspension and the second vehicle, when the solution/suspension and thesecond vehicle are introduced through the inner passage and surroundingpassage in question.

It will be appreciated that, where necessary, such apparatus mayadditionally comprise means for the collection of the particulateproduct, for example, means, such as a filter, for the retention of theproduct in the particle formation vessel, thus to reduce loss of theproduct together with the supercritical solution also formed. Analternative particle collection means may involve a cyclone separatingdevice.

The apparatus may include means for recovering the supercriticalsolution formed on extraction of the vehicles into the supercriticalfluid: means for separating the components of the supercriticalsolution; and optionally means for recycling one or more of thosecomponents back into the apparatus for future use, so as to increase itsoverall efficiency.

It will be further appreciated that the apparatus may comprise more thanone particle formation vessel and/or means for the collection of theparticulate product, thereby allowing for the substantially continuousoperation of the apparatus through simple switching from one particleformation vessel or collection means to another as required. Suchadaptation for continuous operation represents a further embodiment ofthe present invention.

The means for controlling the temperature and pressure in the particleformation vessel preferably comprise an oven and an automatedback-pressure regulator respectively, although ocher appropriate, known,means may be used.

An advantage of apparatus according to the invention is that it canallow particle formation to occur in a completely closed environment,ie. in a closed particle formation vessel. The apparatus can be sealedfrom the atmosphere, making it easy to maintain sterile operatingconditions and reducing the risk of environmental pollution, and it canalso be kept free of oxygen, moisture or other relevant contaminants.The particle formation vessel can also easily be made light-free, ofparticular use for the preparation of photosensitive products such asfor use in the photographic industry.

According to a third aspect of the present invention, there is provideda particulate product made using the method of the first aspect of theinvention and/or the apparatus of the second aspect.

The present invention will now be described, by way of example only,with reference to the accompanying illustrative drawings, in which:

FIG. 1 shows schematically apparatus for use in carrying out a method inaccordance with the first aspect of the invention;

FIGS. 2A and 2B show schematic designs of alternative sets of apparatusfor the same purpose;

FIG. 3 is a longitudinal section through the outlet end of a coaxialnozzle for use in the apparatus of FIG. 1. FIG. 2A or FIG. 2B;

FIGS. 4A and 4B are a longitudinal and a transverse section respectivelythrough the outlet end of an alternative coaxial nozzle for use in theapparatus of FIG. 1, FIG. 2A or FIG. 2B;

FIG. 5 is an SEM (scanning electron microscope) micrograph of lactoseprepared according to Example 1;

FIG. 6 is an XRD (X-ray diffraction) pattern for the sample shown inFIG. 5:

FIG. 7 is an SEM micrograph of lactose prepared according to Example 2:

FIG. 8 is an XRD pattern for the sample shown in FIG. 7;

FIGS. 9 and 10 are SEM micrographs for the product and starting material(maltose) respectively of Example 3;

FIGS. 11 and 12 are XRD patterns for the samples shown in FIGS. 9 and 10respectively;

FIGS. 13 and 14 are SEM micrographs for the product and startingmaterial (trehalose) respectively of Example 4;

FIGS. 15 and 16 are XRD patterns for the samples shown in FIGS. 13 and14 respectively;

FIG. 17 is an XRD pattern for sucrose prepared according to Example 5;

FIG. 18 is an XRD pattern for salmeterol xinafoate prepared according toExample 6.

DETAILED DESCRIPTION

FIGS. 1-4 illustrate apparatus which may be used to carry out a methodin accordance with the present invention, ie. for the formation ofparticles. The subsequent examples illustrate how the invention has beencarried out, in order to prepare particulate products of varioussubstances.

FIGS. 1, 2A and 2B are simplified diagrammatic flow sheets of apparatusof use in carrying out the invention, and FIGS. 3 and 4 show nozzleswhich may be used in such apparatus.

Referring firstly to FIG. 1, the apparatus shown includes a particleformation vessel 6. This is typically a standard reaction vessel, forinstance of the type available from Keystone Scientific Inc., of anappropriate capacity for the particular use to which it is to be put.The temperature and pressure in the vessel are maintained at a constantdesired level, by means of an oven 7 and back-pressure regulator 8 (eg.model number 880-81 of Jasco inc.) respectively.

In use, the system is initially pressurised and stable workingconditions are met. A suitable gas, for example, carbon dioxide, is fedfrom source 1 via conduit 11 to a cooler 2, to ensure liquification, andis fed by conduit 12 to a pump 4. From there it is fed by conduit 13 tothe vessel 6 via a nozzle 20. A solution of a solid of interest, forexample, lactose, in a suitable first vehicle, for example water, isdrawn from source 5 by a conduit 14 to a pump 3 and is fed by conduit 15to the vessel 6 via nozzle 20. A second vehicle, for example ethanol, isfed from source 5 a to nozzle 20 via conduit 15 a and pump 3 a.

The nozzle 20 allows the fluids to be co-introduced into the vessel 6,and may be as shown in FIG. 3 or FIG. 4. The nozzle of FIG. 3 comprisesthree coaxial tubes 30, 40 and 50 which define an inner passage 31, anintermediate passage 41 and an outer passage 51 respectively. Tubes 40and 50 have conically tapering end portions 42 and 52, the angle oftaper of the end portion 52, θ, relative to the main axis of the nozzle,being about 30° in this (non-limiting) example. The end of tube 30 andthe tips of the end portions 42 and 52 define respective orifices 33, 43and 53, with the orifices 43 and 53 being a short distance downstream ofthe orifice 33.

In use of the nozzle, fluids introduced through the inner andintermediate passages 31 and 41 are “pre-filmed” prior to their contactwith a fluid introduced through the outer passage 51. In other words,because of the shapes and dimensions of the nozzle passages, and therelative positions of their outlet orifices, fluids reaching theorifices 33 and 43 are formed into thin fluid films, which films maythen be stretched, and ultimately broken up into individual fluidelements, by a fluid flowing through the outer passage 51.

The nozzle of FIG. 3 allows three fluids to be co-introduced into thevessel 6. For instance, a solution or suspension of the substance ofinterest in the first vehicle may be introduced through the innerpassage 31, the second vehicle through the intermediate passage 41 andthe supercritical fluid through the outer passage 51. “Mixing” of thesolution or suspension and the second vehicle, and their filming, thenoccurs immediately prior to their dispersion by the supercritical fluidbetween orifices 43 and 53. It is also possible to add through one ofthe three passages a desired carrier or other additive intended to formpart of, or be mixed with, the final particulate product. The additiveis then dispersed simultaneously with the substance of primary interest.Also, in situ reactions may be carried out immediately prior to, or on,dispersion by the supercritical fluid, by introducing two or morereactants in two separate vehicles through two of the nozzle passages,the reaction occurring at the passage outlets either immediately priorto, or on, dispersion.

In the nozzle shown, inner tube 30 has an internal diameter of 0.25 mm;intermediate tube 40 has an internal diameter of 0.53 mm; and outer tube50 has an internal diameter of 0.8 mm and an outside diameter of 1.5 mm.The tip opening (53) has an internal diameter of 0.2 mm. The tubes areall made of stainless steel.

However, the nozzle may be made of any appropriate material and have anysuitable dimensions. For instance, the internal diameters of the tubesmay be in the ranges 0.05-0.35 mm (inner); 0.25-0.65 mm (intermediate);and 0.65-0.95 mm (outer), preferably between 0.1 and 0.3 mm (inner); 0.3and 0.6 mm (intermediate); and 0.7 and 0.9 mm (outer). The tip opening53 is likely to have an internal diameter in the range 0.1-0.3 mm,preferably between 0.18 and 0.25 mm.

An alternative form of coaxial nozzle, again for use in the apparatus ofFIGS. 1 and 2 and which again causes “pre-filming” fluids in its innerpassages, is shown in FIGS. 4A and 4B. This nozzle has three coaxialpassages, an inner 61, an intermediate 71 and an outer 81. Their outletorifices are, respectively, 63, 73 and 83, orifices 63 and 73 being ashort distance upstream, in use, of the orifice 83. The in-use directionof fluid flow is indicated by the arrows.

The inlet to the intermediate passage 71 is modified by the inclusion ofa T-shaped cube 89, which allows the introduction into the passage oftwo separate fluids, labelled Fluid A and Fluid B in FIG. 4A. These maybe, for instance, (A) a solution/suspension of a substance in a firstvehicle and (B) a second vehicle, in which case a supercritical fluidmight then be introduced through passages 61 and 81 of the nozzle.Alternatively, Fluids A and B might each be a solution or suspension ofa reactant material, allowing the in situ (ie. in nozzle passage 71)formation of a desired product of the two reactants, which product canthen be produced in particulate form on contact with a supercriticalfluid introduced through outer passage 81. Clearly, the use of such amodified nozzle greatly increases the versatility of the apparatus ofFIGS. 1 and 2, allowing a variety of different fluid types to beintroduced into the particle formation vessel in different combinations.

The dimensions for the FIG. 4 nozzle are as described above in relationto the nozzle of FIG. 3.

Using the apparatus of FIG. 1, the supercritical fluid may be fed underpressure (ie. at a high flow rate when compared with the flow rates ofthe solution/suspension and the second vehicle) through for example theouter nozzle passage 51 of the nozzle shown in FIG. 3 (81 in FIG. 4) thesolution or suspension of the solid of interest in a first vehicle maybe simultaneously fed under pressure through the inner passage 31 (61);and a second vehicle may be introduced through the intermediate passage41 (71). It is believed that the high velocity supercritical fluidemerging from the passage 51 causes the fluids emerging from the ends ofpassages 31 and 41 to be broken up into fluid elements (eq. droplets)from which the vehicles are substantially simultaneously extracted bythe supercritical fluid to result in the formation of particles of thesolid previously held in the first vehicle. It is to be understood,however, that although it is believed that this is what occurs, we donot wish to be bound by this theoretical explanation, and the actualphysical processes occurring may not be precisely as just indicated.

Also, although a configuration has been described in which thesupercritical fluid passes through the outer passage 51, the firstvehicle through inner passage 31 and the second vehicle throughintermediate passage 41, the configuration may be altered ifappropriate, and any one of the three passages, of the nozzle of eitherFIG. 3 or FIG. 4, may be used to carry any one of a number of desiredfluids. Similarly, a nozzle having a different number of coaxialpassages, or of a different overall type, may be used in the FIG. 1apparatus.

The nozzle 20 ensures dispersion of the vehicles by the shearing actionof the high velocity supercritical fluid, and also thorough mixing ofthe dispersed vehicles with the supercritical fluid which simultaneouslyextracts the vehicles from the dispersed liquid, resulting insubstantially immediate particle formation of the solid of interest.Because, using the FIG. 3 or FIG. 4 nozzle, the supercritical fluid andvehicles are introduced coaxially; and dispersion occurs substantiallysimultaneously with vehicle extraction, at the entrance to a vessel inwhich temperature and pressure are carefully controlled, a very highdegree of control is possible of the conditions (e.g. pressure,temperature and flow rates) affecting particle formation at the exacttime when it occurs. The coaxial introduction also allows both vehiclesto be extracted into the supercritical fluid together, even if the firstvehicle is substantially insoluble in the supercritical fluid.

The particles formed are retained in the particle formation vessel bycollecting means 21. The resultant supercritical solution is fed byconduit 16 to a back-pressure regulator 8 and is then fed by conduit 17to a separation vessel 9 where it expands to cause the supercriticalfluid to separate as a gas from the liquid vehicles. The gas may be fedby conduit 18 to a tank 10 and returned by conduit 19 to the cooler 2.The vehicles may also be collected for subsequent re-use. Means, notshown, may be provided to smooth the fluid flow pulse produced by pumps3, 3 a and 4, so as to eliminate, or at least reduce, any flowpulsations.

When sufficient particle formation has occurred in the vessel 6, it isflushed through with clean, dry supercritical fluid, so as to ensureremoval of any residual vehicle. The vessel can then be depressurisedand the particulate product removed.

The alternative apparatuses shown schematically in FIGS. 2A and 2B arefor use in continuous particle formation. That shown in FIG. 2A includestwo particle formation vessels 6 a and 6 b, each of the type shown inFIG. 1 and each including an inlet nozzle 20, as described above, and aparticle collecting means (such as a filter) 21. Oven 7 andback-pressure regulator 8 serve both vessels.

In the apparatus of FIG. 2A, valve A controls the supply of thesupercritical fluid, the first vehicle (containing the substance ofinterest) and the second vehicle to the two vessels 6 a and 6 b, andone-way valves E and F control the outlets from the two vessels to theback-pressure regulator 8. Valves D and G control the supply of the twovehicles to valve A. Valves B and C are needle valves, and items 80 and81 are vents.

The apparatus may be “continuously” operated as follows. Valve A isfirstly set to supply fluids to vessel 6 a, in which particle formationis allowed to occur, as described in connection with FIG. 1. Valve E isset so that the resultant supercritical solution may drain from vessel 6a to the back-pressure regulator 8 for subsequent recycling.

When sufficient particle formation has occurred, valves D and G areclosed to stop the flow of vehicles, whilst the supercritical fluidcontinues to flow through vessel 6 a to dry (flush) the product. Valve Ais then set to supply fluids to the empty vessel 6 b, and valves D and Gre-opened, whilst valve B is opened so as slowly to depressurise vessel6 a. One-way valve E eliminates any chance of a back-flow from vessel 6b or of disruption of the particle formation process now occurring invessel 6 b. Vessel 6 a is removed for collection of the product, andthen refitted and re-pressurised ready for re-use. Supercriticalsolution drains from vessel 6 b via valve F, which is set appropriately.

Once particle formation in vessel 6 b is complete, the valves are setback to allow it to continue in vessel 6 a, whilst 6 b is flushed andemptied. In this way, particle formation in the apparatus can continueuninterrupted.

The apparatus shown in FIG. 2B includes only one particle formationvessel 6, which does not contain any particle collecting means, and twoparticle collection vessels 25 a and 25 b downstream of vessel 6. Inuse, the supercritical solution carries the formed particles to thecollection vessels 25 a and 25 b.

The apparatus also includes an inlet nozzle 20, preferably as describedabove, two vents 26, a back pressure regulator 27, an oven 7 and valvesA−H. Supercritical fluid, solution or suspension (of substance in firstvehicle) and second vehicle are fed to the nozzle 20 where shown.

The apparatus might be used as follows. Initially, (valves C, D, E and Fclosed) the system is pressurised and stable working conditions are met;valves B and H are then closed, driving the flow of supercritical fluidthrough valve A only. The supercritical fluid, the solution/suspensionof the first vehicle and substance of interest and the second vehicleare introduced into vessel 6 and the particles formed are transported bythe resultant supercritical solution, via valve A, to collection vessel25 a which contains a particle retention device. The retention device isplaced at the outlet of the vessel to ensure maximum collection volume.The solid-free supercritical solution (the supercritical fluid and thevehicles) flows across valve G to the back pressure regulator 27. Onemerging from the back pressure regulator the supercritical solutionexpands into a large pressure resistant vessel (not shown), where thevehicles separate from the gas and both can be recycled.

When the collection vessel 25 a is full, switching takes place, closingvalves A and G and simultaneously opening valves B and H. This allowsthe flow of the supercritical solution, emerging from vessel 6, into thesecond collection vessel 25 b. Valves C and G are opened after flowswitching to ensure a high flow of supercritical fluid to flush the fullcollection vessel 25 a, i.e. the supercritical solution volume isreplaced by a supercritical fluid volume. It is estimated that 1-2 timesthe volume of the collection vessel, of supercritical fluid, ensures adry powder. The flushing time is generally short owing to the fact thatthe particles themselves are occupying the volume of the collectionvessel. After flushing, valves C and G are closed and valve F (a needlevalve) is slowly opened to depressurise the full collection vessel 25 a.Since the particulate product takes up the vessel volume only a smallamount of supercritical fluid is discharged, mainly the internal volumeof the fittings involved.

The full collection vessel 25 a is removed and the dry powder collected.After refitting and depressurising via valve C, the vessel is ready forre-use as soon as the second collection vessel 25 b, which has meantimebeen collecting product from vessel 6, is full.

The benefits of using the apparatus of FIG. 2B include:

-   1. The elimination of depressurising and pressurising steps of the    particle formation vessel every time product is collected. This    could mean considerable reductions in the amounts of fluids being    discharged, in particular when using a large volume particle    formation vessel (scaling up) or expensive high purity fluids.-   2. Significant time saving during the flushing (drying) procedure.    In a batch particle formation process only a rather small volume of    the reaction vessel is occupied by the product and the remaining    volume (where dispersion takes place) is taken up by the    supercritical solution. This mixture will eventually be replaced by    at least the same volume of supercritical fluid in the flushing    procedure, which can therefore take a long time when scaled up.-   3. The environment and workers are less exposed to the products    during the recovery step. It can be difficult to collect products    directly from a large reaction vessel due to handling inconvenience    or because the products of interest are light, oxygen or humidity    sensitive which might affect their characteristics or purity.

It is to be understood that the apparatus of either FIGS. 2A or 2B may,be used to carry out the method of the present invention.

The invention will now be further illustrated by the followingnon-limiting examples.

EXAMPLES

The following examples were carried out using a method according to thepresent invention, and apparatus generally similar to chat shown inFIGS. 1-4. They illustrate the versatility of the method of theinvention, its usefulness in forming particles of materials which wouldotherwise be difficult to prepare by supercritical fluid techniques andthe advantageous effects which can thereby be achieved.

Examples 1 & 2— Formation of Lactose Particles

The following examples illustrate the successful and controlledformation of crystalline lactose, using carbon dioxide as asupercritical fluid, despite the very low solubility of lactose inconventional CO₂-soluble organic solvents. According to the presentinvention, two vehicles were used for the lactose, water as the firstand an organic solvent (methanol), which is miscible with water andsoluble in supercritical carbon dioxide, as the second.

Example 1— Preparation of Lactose (I)

In accordance with the invention, a solution of lactose in a relativelysmall amount of water and a relatively large amount of a second vehicle,methanol, was used. The solution was co-introduced, with supercriticalCO₂, into a particle formation vessel of the type shown in FIGS. 1 and2, through a three-passage nozzle of the type shown in FIG. 3. Thepressure and temperature inside the vessel were carefully maintained atthe desired operating levels throughout particle formation. It isthought that the miscible water and methanol were extracted togetherinto the supercritical CO₂, despite the insolubility of water an in thesupercritical fluid.

0.3 g of alpha-lacrosse monohydrate was dissolved in 2 ml de-ionisedwater, 98 ml of methanol was added to the aqueous solution and themixture was introduced into the 32 ml particle formation vessel throughthe three-passage nozzle. The operating conditions were 270 bar and 70°C. inside the vessel, a solution flow rate (in the intermediate nozzlepassage) of 0.5 ml/min and a supercritical CO flow rate (in the innerand outer passages) of 7.5 ml/min. The product (a fine white powder) wascollected at the end of the experiment. An SEM micrograph and XRDpattern for the product are shown in FIGS. 5 and 6 respectively.

Example 2— Preparation of Lactose (II)

In an experiment similar to that of Example 1, a 0.5% w/v solution ofalpha-lactose monohydrate in methanol:water (95:5 v/v) was prepared anddelivered to a 50 ml high pressure particle formation vessel via atwo-passage nozzle. The working conditions were 150 bar and 50° C.inside the vessel, with a flow rate of 0.7 ml/min for the solution and 9ml/min for the supercritical CO₂. The collected product was a freeflowing, fine white powder. FIGS. 7 and 8 show an SEM micrograph and XRDpattern respectively for this product.

The SEM micrographs for the products of Examples 1 and 2 reveal a markeddifference in the shape of the lactose particles prepared under thedifferent operating conditions. The XRD patterns indicate thecrystalline nature of the products.

As can be seen from these examples, the present invention provides anextremely effective technique for the controlled formation of lactoseparticles using supercritical fluids, without loss of control over thesize, shape, form and other properties of the resultant particles. Thisis achieved despite the fact that lactose has a very low solubility inmany conventional organic solvents which would themselves be soluble insupercritical carbon dioxide, which has meant that previously it wouldnot have been possible to precipitate lactose using supercritical carbondioxide.

Examples 3-5— Preparation of Other Sugars

These examples illustrate the preparation of other sugars, which likelactose would be difficult to produce in particulate form usingconventional methods.

The experiments were carried out using apparatus of the type shown inFIGS. 1 and 2, with the three-passage nozzle of FIG. 3. In accordancewith the preferred version of the invention, a solution of the desiredsugar in a first vehicle (water) was introduced into the nozzleseparately from the second vehicle (ethanol), in which the sugar issubstantially insoluble but which is itself soluble in supercritical CO.The solution and the second vehicle came into contact only immediatelyprior to their dispersion by the supercritical fluid.

Note that in Examples 3 and 4, the product sugars had an amorphousnature, despite having been prepared from crystalline startingmaterials. Amorphous products have great advantages for use as carriersfor pharmaceuticals, in that they have a relatively high surfacearea—they can therefore carry more of the pharmaceutical and can alsodissolve more quickly than the equivalent crystalline forms. Theirsmooth surfaces facilitate the release of the carried pharmaceutical,making them ideal for delivery of drugs by, for instance, inhalationmethods.

Using known particle formation methods, it is very difficult, and oftenvery expensive, to produce such sugars in amorphous form. Examples 3-5therefore highlight a highly advantageous application for the method ofthe present invention.

They also demonstrate the use of the invention to manipulate the shapeand degree of crystallinity of particulate sugars. Such manipulation isdifficult, if not impossible, to achieve using conventionalcrystallisation methods.

Example 3— Preparation of Maltose

1.01 g of maltose monohydrate (Sigma UK) was dissolved in 5 ml ofde-ionised water and introduced into a 32 ml particle formation vesselthrough the intermediate nozzle passage, at a flow rate of 0.03 ml/min.The vessel was maintained at 250 bar and 70° C. Absolute ethanol wasco-introduced into the vessel through the inner nozzle passage, at arate of 0.4 ml/min, and supercritical CO₂ through the outer passage at arate of 9 ml/min. A free-flowing white powder was collected.

The SEM photographs for the product (FIG. 9) and the starting material(FIG. 10) show a remarkable difference in crystal habit between the twosolids. The product is in the form of spongy spheres with smoothsurfaces; its XRD pattern (FIG. 11) reveals its amorphous naturecompared with the crystalline nature of the maltose used as the startingmaterial (see the XRD pattern in FIG. 12).

Example 4— Preparation of Trehalose

Trehalose is another sugar used as a carrier for pharmaceuticals and toprotect proteins from unfolding under certain processing conditions. Itscontrolled preparation in particulate form is therefore highlydesirable.

In this experiment, 1.01 g of alpha, alpha-trehalose dihydrate Sigma(UK) was dissolved in 4 ml of de-ionised water and introduce a into a 32ml particle formation vessel through the intermediate nozzle passage ata flow rate of 0.015 ml/min. The vessel was maintained at 250 bar and70° C. Absolute ethanol was co-introduced into the vessel through theinner nozzle passage, at a rate of 0.4 ml/min, and supercritical CO₂through the outer passage at a rate of 8 ml/min. A free-flowing whitepowder was collected.

The SEM micrographs for the product (FIG. 13) and the starting material(FIG. 14) again show a considerable difference in crystal habit betweenthe two solids. The product is in the form of spongy, smooth-surfacedparticles with an enhanced surface area. The XRD pattern for the product(FIG. 15) reveals its amorphous nature compared with the crystallinestarting material (FIG. 16).

Example 5— Preparation of Sucrose

2.04 g of sucrose (Sigma UK) was dissolved in 10 ml of de-ionised waterand introduced into a 32 ml particle formation vessel through theintermediate nozzle passage, at a flow rate of 0.02 ml/min. The vesselwas maintained at 250 bar and 60° C. Absolute ethanol was co-introducedinto the vessel through the inner nozzle passage, at a rate of 0.35ml/min, and supercritical CO₂ through the outer passage at a rate of 8ml/min. A free-flowing white powder was collected.

The XRD pattern for the product (FIG. 17) reveals a crystalline nature.

Example 6

w/v solution of salmeterol xinafoate in acetone was mixed at a rate of0.1 ml/min with 0.4 ml/min of n-hexane (antisolvent) prior tointroduction to the particle formation vessel through the intermediatepassage of the three-component nozzle as in FIG. 4A. Supercritical CO₂was fed at a rate of 15 ml/min through the inner and outer passages todisperse the magma (acetone solution, hexane and the continuouslygrowing salmeterol nuclei/embryos) and extract the acetone-hexanesolvent mixture. The particle formation vessel was maintained at 150 barand 60° C. A fine, free-flowing powder product was collected andexamined by XRD. FIG. 18 shows the XRD pattern which indicates acrystalline polymorph I of the drug material.

This result demonstrates the effectiveness of these methods andapparatus to influence crystallisation by addition of antisolventsbefore dispersion by the supercritical fluid. The opportunity isprovided to control nucleation and growth of particles by controllingthe relative rate of addition of the solution of the material ofinterest to the antisolvent, or vice versa.

Example 7—

Preparation of Protein Particles

In this example, the method of the invention was used to prepare thewater-soluble protein R-TEM beta-lactamase, again using two vehicles. Anaqueous protein solution was co-introduced into a particle formationvessel of the type shown in FIGS. 1 and 2 with a second vehicle,ethanol, which is both miscible with water and soluble in supercriticalCO₂. The two fluids were introduced, with the supercritical CO₂, througha three-passage nozzle of the type shown in FIG. 3, in such a way thatcontact between the aqueous solution and the ethanol, dispersion of thesolution and the ethanol and extraction of the water and the ethanol alloccurred very close together in time. It is thought that the aqueoussolution and the ethanol “mixed” on contact, and that the water andethanol were extracted together into the supercritical CO₂, despite theinsolubility of water in the supercritical fluid.

A 0.25% w/v solution of R-TEM beta-lactamase (kindly provided by theCentre for Applied Microbiology, Porton Down, Salisbury SP4 0JG, batchnumber 1TEM1L88) in deionised water was fed to the 32 ml particleformation vessel via the inner passage of the three-passage nozzle, at aflow race of 0.04 ml/min. Absolute ethanol was co-introduced through theintermediate nozzle passage at a rate of 0.4 ml/min and supercriticalCO₂ through the outer passage at a rate of 8 ml/min.

Here, the method of the invention, and the use of a three-passage nozzleallowed the aqueous protein solution to be mixed with the ethanolimmediately prior to impression of the two vehicles by the supercriticalfluid. The contact time between the aqueous and the organic fluids wasso short that the risk of protein unfolding or denaturing wasminimal—another advantage of using the present invention to prepareproteins and other active products.

The particulate product formed retained substantial enzymatic activitywhen tested colorimetrically using the chromogenic cephalosporinNitrocefin (Oxoid, Unipath Limited, Basingstoke, Hampshire, England) andthe assay method of O'Callaghan (C. H. O'Callaghan, A. Morris, S. Kirbyand A. H. Shingler, Antimicrobial Agencs and Chemotherapy, 1, pp 283-288(1972)). This illustrates the successful use of the method and apparatusof the invention in preparing particulate protein products in acontrolled manner, even where the proteins are insoluble in, orincompatible with, organic solvents.

The above examines show how the apparatus and method of the presentinvention can be used to produce particulate products of various typesin a highly controlled manner, without the usual solvent constraints.Applications of the invention might include for instance:

producing controlled size and shape particles of products for use in thepharmaceutical, photographic, ceramics, explosives/propellants,dyestuffs and food industries and others, especially of products whichdecompose or are otherwise compromised when subjected to conventionalparticle formation and milling techniques.

producing solid, stable forms of molecules and macromolecules which aredifficult to process or freeze dry (e.g. proteins, peptides and polymersgenerally).

producing a particular polymorphic form of a compound or separatingand/or enriching mixtures of isomers (including optical isomers) orpolymorphs.

purifying drugs and other products, by removal of trace impurities(including solvents) using controlled selective precipitation (eg. usingthe invention to precipitate the impurities themselves).

coating substrates in a controlled manner, including with thin filmliquid coatings.

controlling “doping” of compounds in products based on crystal lattices,or producing intimate blends of two or more products, such as oneproduct within a matrix of another, or one product coated onto or coatedwith another.

preparing completely new phases or materials under conditions notachievable using conventional particle formation techniques.

1-29. (canceled)
 30. A method for forming a particulate product,comprising: maintaining a predetermined temperature and a predeterminedpressure within a particle formation vessel; introducing a supercriticalfluid into the particle formation vessel; introducing a first vehiclecontaining at least one substance into the particle formation vessel,wherein the first vehicle is substantially insoluble within thesupercritical fluid; introducing a second vehicle into the particleformation vessel, wherein the second vehicle is substantially solublewithin the supercritical fluid; dispersing and extracting the first andsecond vehicles by the supercritical fluid immediately andsimultaneously upon introducing the first and second vehicles and thesupercritical fluid into the particle formation vessel; and forming aplurality of particles containing the at least one substance.
 31. Themethod of claim 30, wherein the first vehicle containing the at leastone substance is a solution, a suspension or a combination thereof. 32.The method of claim 31, wherein a combined mixture of the first andsecond vehicles is soluble within the supercritical fluid.
 33. Themethod of claim 32, wherein the supercritical fluid contains a compoundselected from the group consisting of carbon dioxide, nitrous oxide,sulfur hexafluoride, xenon, ethylene, chlorotrifluoromethane, ethane,trifluoromethane, derivatives thereof and combinations thereof.
 34. Themethod of claim 32, wherein the first vehicle is water and thesupercritical fluid is carbon dioxide.
 35. The method of claim 31,wherein the at least one substance contains a pharmaceutical agent. 36.The method of claim 35, wherein the particles contain the pharmaceuticalagent coated with a taste-masking agent.
 37. The method of claim 31,wherein the second vehicle has a greater flow rate into the particleformation vessel than the first vehicle containing the substance. 38.The method of claim 37, wherein the supercritical fluid contains one ormore modifiers.
 39. The method of claim 31, wherein the first vehiclehas functional groups that interact through hydrogen-bonding ordipole-dipole interactions with functional groups contained on thesecond vehicle.
 40. The method of claim 31, wherein the at least onesubstance and the first vehicle are substantially polar and the secondvehicle is substantially non-polar.
 41. The method of claim 31, whereinthe at least one substance and the first vehicle are substantiallynon-polar and the second vehicle is substantially polar.
 42. The methodof claim 31, wherein the at least one substance is substantiallyinsoluble in the second vehicle.
 43. The method of claim 31, wherein thefirst vehicle is water and the supercritical fluid is carbon dioxide.44. The method of claim 43, wherein the second vehicle contains analcohol.
 45. The method of claim 44, wherein the at least one substancecontains a protein, a sugar, a derivative thereof or a combinationthereof.
 46. The method of claim 44, wherein the at least one substancecontains a material selected from the group consisting of lactose,maltose, trehalose, sucrose, salmeterol xinafoate, beta-lactamase, awater-soluble sugar, a water-soluble protein, a derivative thereof and acombination thereof.
 47. The method of claim 46, wherein the at leastone substance contains beta-lactamase and the second vehicle containsethanol.
 48. The method of claim 46, wherein the at least one substancecontains lactose, maltose, trehalose or sucrose and the second vehiclecontains ethanol.
 49. The method of claim 31, wherein the first vehiclecontains a polar organic solvent and the supercritical fluid containscarbon dioxide.
 50. The method of claim 49, wherein the first vehiclecontains acetone.
 51. The method of claim 49, wherein the second vehiclecontains a non-polar organic solvent.
 52. The method of claim 51,wherein the second vehicle contains hexane.
 53. The method of claim 51,wherein the at least one substance contains a protein, a sugar, aderivative thereof or a combination thereof.
 54. The method of claim 51,wherein the at least one substance contains salmeterol xinafoate or aderivative thereof.
 55. The method of claim 54, wherein the firstvehicle is acetone and the second vehicle is hexane.
 56. A method forforming a particulate product, comprising: maintaining a predeterminedtemperature and a predetermined pressure within a particle formationvessel; flowing a supercritical fluid, a first vehicle containing atleast one substance and a second vehicle into the particle formationvessel, wherein the first vehicle is substantially insoluble within thesupercritical fluid and the second vehicle is substantially solublewithin the supercritical fluid; dispersing and extracting the first andsecond vehicles by the supercritical fluid immediately andsimultaneously upon combining the first and second vehicles and thesupercritical fluid within the particle formation vessel; and forming aplurality of particles containing a pharmaceutical agent coated with ataste-masking agent, wherein the supercritical fluid contains carbondioxide.
 57. The method of claim 56, wherein the first vehiclecontaining the at least one substance is a solution, a suspension or acombination thereof.
 58. The method of claim 57, wherein a combinedmixture of the first and second vehicles is soluble within thesupercritical fluid.
 59. The method of claim 58, wherein the firstvehicle is water.
 60. The method of claim 57, wherein the second vehiclehas a greater flow rate into the particle formation vessel than thefirst vehicle containing the substance.
 61. The method of claim 60,wherein the supercritical fluid contains one or more modifiers.
 62. Themethod of claim 57, wherein the at least one substance is substantiallyinsoluble in the second vehicle.
 63. The method of claim 57, wherein thefirst vehicle is water.
 64. The method of claim 63, wherein the secondvehicle contains an alcohol.
 65. The method of claim 57, wherein thefirst vehicle is a polar organic solvent.
 66. The method of claim 65,wherein the first vehicle is acetone.
 67. The method of claim 65,wherein the second vehicle contains a non-polar organic solvent.
 68. Themethod of claim 67, wherein the second vehicle contains hexane.